1. Field of the Invention
The present invention relates to digital to analog converters, and more particularly to a digital to analog converter using pulse width modulation and adopted for use with computer monitors, and to the method for controlling the same.
2. Background Art
Generally, a data input device and a data processing unit of a computer peripheral are provided to deal with digital data. Once the user of a device operates a key or button, digital data is generated in the input device and the data is supplied to the data processing unit or microcontroller to produce output digital data for driving actuators or for controlling operation of the device. The output digital data often requires controlling analog circuits and other system elements with an analog voltage or current. To satisfy this requirement, digital to analog converters (DACs) have been used. U.S. Pat. No. 4,532,496 to Kazuaki Ichinose entitled Pulse Width Modulation Type Digital To Analog Converter describes one example of a known type of DAC.
Another example of a DAC is typically implemented by a number of binary weighted resistors supplied with a reference voltage through digitally controlled electronic switches, in which an analog voltage is produced proportional to the digital input. The DAC can be made up of a pulse generator and an integrator where the pulse generator produces a pulse signal corresponding to the digital input and through the integrator the pulse signal is converted into an analog voltage signal. Such a DAC is exemplified by FIG. 1, for use in a computer monitor. The monitor has a control panel which has a plurality of keys, or buttons, 10 for controlling horizontal size, horizontal position, vertical size, vertical position of an image by pressing the corresponding buttons. A key signal of a key 10 is supplied to a microcontroller 20 which produces a digital signal having a control value Din corresponding to a key input. Then a pulse generator 30 receives the digital signal Din and generates a pulse signal having a pulse width Tw proportional to the digital input value. The output pulse signal of pulse generator 30 is supplied to an integrator 40 which converges the input pulse signal on a DC voltage value proportional to the duty ratio of the pulse signal.
In FIG. 2, there is shown one example of integrator 40 which has a resistor R1 and a capacitor C1. Also, an example of the output voltage signal Vout produced by integrator 40 is shown in FIG. 3. In FIG. 3, the output voltage Vout can be expressed by the following: ##EQU1## where Tw is a width of the input pulse signal and T is the pulse period, and Vcc represents the voltage level of the input pulse.
The output voltage signal Vout of integrator 40 is supplied to a horizontal/vertical output circuit 50 to control the horizontal/vertical size or position of the display image, which are controlled in proportion to the voltage level of the output signal Vout.
FIG. 4 illustrates the analog output signal of the integrator 40, occurring in the form of a ripple current at the steady operation period. The ripple of the analog output will function as a noise. Further, FIGS. 5 and 6 illustrate both the magnitude of the ripple and the converging speed of the analog output signal occurring during the transient period until it reached the steady state.
Referring to FIG. 5, if the time constant of the integrator is set to a relatively low value, the converging speed becomes fast, however it can be seen that the magnitude of the ripple becomes large accordingly. Conversely, at FIG. 6, if the time constant of the integrator is set to relatively high value, the magnitude of the ripple becomes small but the problem is that the converging speed becomes slow, which result in lowering of system response speed.